1. Field of the Invention
The invention relates to a power supply circuit, and particularly to a power supply circuit for an audio apparatus having a switching type DC--DC converter.
2. Description of the Related Art
It is conventionally known that an excess current caused by a short circuit of a load or the like results in damage of an internal component of a DC--DC converter which constitutes a power supply circuit. An example of a configuration for eliminating such a drawback is disclosed in Japanese Utility Model Publication (Kokai) No. SH062-161587.
FIG. 6 shows a DC--DC converter having an overcurrent detecting circuit disclosed in the above-identified publication. Referring to the figure, a DC--DC converter 6 functions as a power supply for driving a load 74.
An overcurrent detecting section comprises a detecting resistor 71, a reference voltage 4, an overcurrent detecting circuit 72, and an operation stop signal generating circuit 73. The overcurrent detecting circuit 72 compares a voltage drop produced by a current flowing through the detecting resistor 71 inserted in series into an input line, with the reference voltage 4. When the voltage drop is larger than the reference voltage 4, it is determined that the circuit is in an overcurrent condition, and a determination signal is sent to the operation stop signal generating circuit 73. In response to the detection signal indicative of the overcurrent condition, the operation stop signal generating circuit 73 supplies an operation stop signal for stopping the operation of the DC--DC converter 6 to the DC--DC converter 6.
Japanese Utility Model Publication No. SH062-161587 also discloses protection processing for an excess current by utilizing a PWM control IC which is generally used for controlling an output voltage of a switching type DC--DC converter.
Specifically, the circuit is protected in the following manner. When an excess current is produced, the ON duty period of a PWM signal is set to be 0, so as to stop the switching operation of the DC--DC converter.
Such a DC--DC converter is widely used in the field of a sound apparatus. In a power supply circuit for an amplifier which is mounted on a vehicle and used for driving a loudspeaker, particularly, a high output cannot be obtained by using a voltage of a battery (a power supply voltage), and hence the power supply voltage is boosted by the DC--DC converter so that the boosted voltage is used as a power supply for the amplifier.
If a loudspeaker having an impedance which is smaller than a loudspeaker impedance range assumed for the amplifier is used, the above-mentioned overcurrent protecting circuit tries to prevent an excess current from flowing through a switching device and a transformer.
In the above-described protecting circuit, since the switching operation of the DC--DC converter is stopped, also the operation of the amplifier is stopped. Accordingly, the loudspeaker is not driven, and a dropout of reproduced sound occurs.
The detecting resistor 71 which constitutes the overcurrent detecting section is inserted in series into the input line dedicated to the detection of an excess current. Accordingly, an electric power loss due to the overcurrent detection is produced.